The present invention generally relates to optical networks. More specifically, the present invention relates to a bi-directional path switched ring (or multiple interconnected rings) that determines and switches only failed optical signals in an optical network.
One typical optical network configuration currently used for transporting and protecting traffic is a bi-directional line switched ring (BLSR) network. In a typical embodiment of a BLSR network, two fibers are used to carry traffic. More specifically, one fiber is used to carry working traffic in one direction and another fiber is used to carry protection traffic in the reverse direction.
FIG. 1 is a simplified diagram illustrating a conventional two-fiber BLSR network 100. Working traffic is carried on the optical fibers 102a–f in the clockwise direction, and protection traffic is carried on the optical fibers 104a–f in the counterclockwise direction.
Each node within the BLSR network 100 is generally responsible for detecting transmission problems on an optical fiber. An optical fiber pair, for example, optical fibers 102a and 104a, is more commonly called a link or line. Since a link typically connects two adjacent nodes, the two nodes respectively located at both ends of the link usually detect a transmission problem relating to a link. Once a transmission problem is detected, the two nodes then take appropriate actions to re-route or switch all the traffic intended for transmission on the problematic link via alternate link(s). FIG. 2 illustrates the switching of failed paths in the case of a link failure in the BLSR network 100.
Referring to FIG. 2, for example, if a problem is detected with the link (optical fibers 102b and 104b) connecting nodes 2 and 3, nodes 2 and 3 then re-route all the working traffic via the protection path formed by the optical fibers 104a,f,e,d,c. Hence, all the working traffic originally intended for transmission via the optical fiber 102b is now re-routed to node 3 via nodes 1, 6, 5, 4 and 3 using optical fiber 104a,f,e,d,c. 
The foregoing re-routing scheme used by the BLSR network 100 may use spare or protection capacity on the optical network inefficiently. This inefficiency may be attributed to the requirement that all the working traffic (or optical signals or paths) on a problematic link be re-routed even though only some of the optical signals being carried on the link are affected. Thus, partial failure of a link (where only some of the wavelengths fail due to, for example, selective failure of non-redundant band wavelength division multiplexing (WDM) modules) may cause traffic on the entire link to be re-routed or switched. This may result in switching optical signals that have not failed. Hence, spare or protection capacity on the network is inefficiently used or wasted.
The inefficient use of spare capacitor can also be attributed to how the BLSR network detects and corrects transmission problems. Since the re-routing or switching of optical signals is initiated and coordinated by the two nodes respectively situated at both ends of the problematic link, optical signals may unnecessarily be re-routed to these two nodes before they reach their intended destination nodes. This inefficiency is commonly referred to as the backhaul problem.
Referring to FIG. 2, for example, assume that optical signals originating from node 2 are to be transmitted to node 4 via node 3 and that a problem occurs with the optical fiber 102b. As a result, optical signals from node 2 are first re-routed through nodes 1, 6, 5 and 4 and then delivered to node 3 via the protection capacity. Node 3 then forwards the optical signals to the destination node, which is node 4, via the link 102c as it had done before optical signals were diverted from the link 102b. As can be seen, optical signals from node 2 are first routed through node 4, which is the destination node 3 unnecessarily and then from node 3 back to node 4 again. Hence, optical signals are redundantly routed through a destination node and back, thereby wasting wavelength capacity on the network. Similarly, the same inefficiency can arise in connection with node 2. For example, a source node, such as node 1, which transmits optical signals via node 2 to node 3 many, due to re-routing caused by a problem on the link 102b, subsequently receives its previously transmitted optical signals.
BLSR networks also require special hardware that optically switches the signals at every node in the ring. This hardware must switch all traffic at the nodes located adjacent to the link failure to route the working traffic via the protection capacity. In addition, all other nodes must pass this protection traffic through. Moreover, switching all optical signals associated with an affected link also makes it very difficult to plan and estimate an optical budget for the optical network.
The above inefficiency problem may be solved by using a bi-directional path switched ring (BPSR) network. In a BPSR network, only failed optical signals or paths within a link are re-routed or switched. Furthermore, the failure is detected and handled by the source and destination nodes relating to the failed optical signal or path thereby reducing the above backhaul problem. FIG. 3 illustrates the switching of optical signals in the case of a link failure in a BPSR network. Referring to FIG. 3, a problem arises in the link between nodes 2 and 3. In contrast to the situation shown in FIG. 2, for working traffic to be delivered from node 1 to node 4 (which, under normal conditions, would have been routed via nodes 2 and 3), such working traffic is re-routed directly to node 4 via nodes 6 and 5 using the protection capacity without going through node 2 first.
It would be desirable to provide a system and method which is capable of determining failed optical signals within a link and switching such signals more efficiently and reliably in a BPSR network and providing a switching architecture that reduces network complexity and cost.